This invention relates to thermoplastic films suitable for manufacturing containers for improving and extending the storage life of meat and other foodstuffs. More specifically, this invention relates to coextruded multilayer thermoplastic films containing volatile antioxidants.
Adding volatile antioxidants into packaging materials is known in the art. For example, this technique has been used in the cereal industry where volatile antioxidants such as butylated hydroxytoluene ("BHT") and butylated hydroxyanisole ("BHA") are added to the packaging materials for cereal liners. One way of accomplishing this is to blend the antioxidant with a thermoplastic polymer which can then be extruded to form a layer of the packaging material.
High levels of volatile antioxidants in food packaging materials cause migration of some of the antioxidant from the packaging material and into the food. The amount of antioxidant used in the packaging material impacts the amount of antioxidant that migrates to the food.
Previously, the amount of volatile antioxidant that could effectively be incorporated into thermoplastic packaging materials was limited because of the tendency of the antioxidant to evaporate from the polymer during extrusion. Additionally, storage of the packaging material itself is a concern because the volatile antioxidants can migrate out of the material during storage, reducing the amount of antioxidant remaining in the packaging material at the time the packaging material is actually used to store foodstuffs.
U.S. Pat. No. 4,880,696 issued to Yanidis ("Yanidis") discloses a method for increasing the amount of antioxidant incorporated into a packaging material produced using an extrusion coating process. Yanidis teaches extruding a thermoplastic composition, comprising an antioxidant-containing layer and a more remote thermoplastic layer, onto a substrate such that the antioxidant-containing layer becomes sandwiched between the substrate and the more remote layer. Since the antioxidant-containing layer becomes sandwiched soon after being extruded, evaporation of the antioxidant is reduced.
The antioxidant-containing layer in Yanidis is exposed to open air for a brief period of time after the layer is extruded and before the layer is coated onto the substrate. This exposure to open air allows some of the volatile antioxidant to escape, reducing the amount of volatile antioxidant left in the material to be available for its intended purpose. Additionally, the more remote thermoplastic layer in Yanidis is also exposed to open air during processing and may also be exposed to open air after processing and during storage before being used in the manufacture of containers. This exposure to open air allows volatile antioxidants to migrate through the more remote thermoplastic layer and escape into the open air, further reducing the amount of volatile antioxidant left in the material to be available for its intended purpose. Thus, a need exists in the industry for further reducing the amount of volatile antioxidant that escapes or evaporates during processing of antioxidant-containing materials.
Additionally, the process of extrusion coating a substrate, as is done in Yanidis, can be too slow for efficient production of some articles made from antioxidant-containing materials. Thus, a need exists in the industry for increasing processing rates for producing such antioxidant-containing materials.